Sizing agents are used by the paper industry to give paper and paperboard some degree of resistance to wetting and penetration by aqueous liquids. There are two basic categories of sizing agents: acid and alkaline. Acid sizing agents are intended for use in acid papermaking systems, traditionally less than pH 5. Analogously, alkaline sizing agents are intended for use in alkaline papermaking systems, typically at a pH greater than 6.5.
Most acid sizing agents are based on rosin. The development of sizing with a rosin-based size is dependent upon its reaction with papermaker's alum, Al.sub.2 (SO.sub.4).sub.3.14-18 H.sub.2 O. Since aluminum species that exist predominantly at a low pH (&lt;pH 5) are required for the appropriate interactions needed to effect sizing, rosin and alum have been used primarily in acid papermaking systems. It has been shown that, by proper selection of addition points in the papermaking system and by using cationic dispersed rosin sizes, rosin-based sizes can be used in papermaking systems up to about pH 7, thus extending the range of acid sizes. However, due to the limitations imposed by alum chemistry, the efficiency of rosin-based sizes decreases above about pH 5.5.
Sizing agents developed for papermaking systems above pH 6.5 are generally based on alkyl ketene dimer (AKD) or alkenyl succinic anhydride (ASA). AKD sizes function by forming covalent bonds with cellulose to give proper orientation and anchoring of the hydrophobic alkyl chains. This covalent bond formation makes AKD sizing very efficient and resistant to strong penetrants. However, AKD sizes have some limitations: small changes in the amount of size added can lead to large differences in sizing (steep sizing response curve), and there is a slow rate of sizing development (cure).
The other major alkaline sizing agent is based on ASA. As with AKD, the development of sizing with ASA sizes is also dependent on the formation of covalent bonds with cellulose to give proper orientation and anchoring. ASA is more reactive than AKD, resulting in the greater development of sizing on-machine. However, the reaction rate with water is also greater, producing a hydrolyzate that is an inefficient sizing agent in alkaline systems and contributes to the formation of deposits on the papermaking machine. To minimize the formation of hydrolyzate, ASA is typically emulsified at the mill immediately before addition to the papermaking system.
Cationic resins have been used previously in the papermaking process, although not the cationic resins of this invention in combination with rosin in the absence of alum or with amounts of alum.ltoreq.0.3%, based on the dry weight of paper pulp. For example, U.S. Pat. No. 3,186,900 discloses sizing paper at a neutral to acid pH using alum and a preblend of a rosin size and a cationic polyamide-epihalohydrin resin, which is not one of the cationic resins of this invention. U.S. Pat. No. 3,966,654 discloses an aqueous fortified rosin dispersion consisting essentially of (a) 5% to 50% fortified rosin, (b) 0.5% to 10% of a cationic resin dispersing agent selected from (i) a polyaminopolyamide-epihalohydrin resin, (ii) an alkylenepolyamine-epihalohydrin resin and (iii) a poly(diallylamine)-epihalohydrin resin, and (c) water to 100%. Papermaking examples disclose the use of at least 0.4% alum. A process for the nonreactive sizing of paper at a neutral pH in the substantial absence of alum is disclosed in U.S. Pat. No. 4,878,999. The sizing composition comprises saponified fortified rosin and a cationic material consisting essentially of a cationic polymer and a cationic starch.
Rosin sizing with polyamine mordants at a pH of 3 to 10 with and without alum is disclosed in Biermann, C. J., Tappi Journal, May, 1992. Quaternary amines are said to produce very little sizing.
There is still a need for a rosin sizing system for use at a neutral to alkaline pH that does not have the disadvantages of ASA and AKD sizes.